In vitro reactions of bundles of fibers with diameters 20-500 μm and crushed glasses of fractions 500-800 μm were compared with the reactions of plates of the same bioactive glasses. The samples were immersed in simulated body fluid (SBF) for 2-7 days. After immersion the changes on the surfaces of the samples were observed by SEM/EDXA. Layer formation on the glass surface was found to vary with glass composition, sample shape and local condition of single particle/fiber. However, only some fibers or particles formed similar in vitro reaction layers as the plates. The product form did not change the in vitro bioactivity of particles or fibers exposed to the bulk immersion solution. When the glasses were used as fiber bundles or particle beds, the packing degree and the flow of body fluids within the system interfered with the reactivity. Also a clear correlation between in vivo layer formation in bone and in vitro of the glass plates could be found.